Help with voltage regulator/limiter

[BjornM]

I'm stuck with finding a suitable IC solution, maybe someone here can give a suggestion.

I need a voltage limiter. Incoming voltage 15-21 V. Max 10 A. Outgoing voltage should be capped at 18 V, but lower voltages should also pass through. So in other words: incoming is 15-21 V, outgoing should be 15-18 V. Power consumption important, so I can't just put a 18 V zerner diode in parallel and be done with it. And optimally I want the limiter to be adjustable.

AFAIK no regular voltage regulator can do what I need.

Here is a voltage limiter from Maxim, but it gives an ugly sawtooth output at excess voltage, not very attractive.

https://www.maximintegrated.com/en/products/power/supervisors-voltage-monitors-sequencers/MAX6495.html

95% of the time the incoming voltage will be less than 18 V, so I'm thinking of using a MOSFET with very low RDSon, so it passes right through, but once it hits 18 V the transistor should start gradually blocking and burning off excess voltage to keep output at 18 V. Just can't find an IC that can control it this way. And I'm not good enough to trust my own OP-amp designs, will probably have issues with oscillations, etc. The schematic below should work in theory, but I'm not sure I can trust it.


BTW, it is for a solar panel battery charger.

[Ian.M]

You wont find a single IC solution for that that operates as a regulator with a smooth output.  At 21V a linear solution would have to dissipate 30W, which is the domain of beefy power transistors on a large heatsink (+ of course something to control them)

You need some sort of buck converter, rated for 10A out + safety margin and for the max. transient input voltage it could see, with good enough filtering to reduce the ripple to your applications requirements.   With a max sustained input voltage of 21V, it needs to be rated for >50V in, more for an automotive or marine application, (or provide heavy duty unidirectional TVS diode clamping for transients with a working voltage >21V and clamping voltage <50V).  Its desirable for it to have synchronous rectification to minimize voltage drop, otherwise even with a beefy Schottky output diode, you'll loose at least half a volt Vin => Vout drop once the the regulator drops out of regulation with decreasing input.  If you buy one 'off the shelf', it will be a fairly large and moderately expensive module with a sizable heatsink, and certainly not a single chip.   Since you want to use it as a battery charger look for a variable output voltage one with an adjustable current limit, and an indicator LED for whether its in CC or CV mode.

[BjornM]

Thanks Ian,

The input is from a solar panel, and through the charge cycle of my batteries (5s LiFePo4) it will be 10 A. But when they reach absorption voltage (18.13 V) the current is only a fraction. I need this circuit to protect the batteries from over voltage when they already are fully charged. So I'm hoping that I can get away with burning it off as heat.

Was trying to avoid a complex and bulky solution with a DC/DC converter.

[madires]

Why not buying an inexpensive COTS solar charge controller?

[BjornM]

To save money, space and point of failure.

But if I can't figure out a circuit, then that's what I might do.

The thing is, I can connect the panel directly to the battery without any apparent problem. But for a long term solution, I'd like to cut off the overvoltage.

[Ian.M]

You could increase the hysteresis of your MAX6495 in its "Figure 2. Overvoltage-Limiter Protection Switch Configuration" to reduce fast oscillation, by adding a high value resistor between IN and OVSET.  When under the threshold it is effectively in parallel with R1, but once it disconnects, it will lift the voltage at OVSET by a small amount proportional to the excess input voltage, preventing reconnection until either the panel output has dropped to the battery voltage (i.e. nighttime) or the battery voltage has dropped enough.   You'll have to do the math and decide how much discharge you want to accept below fully charged before reconnection.

[bdunham7]

Your circuit should probably work OK for that application, but you will need reverse current flow protection so that your battery doesn't discharge through the solar panels--unless your panel already has a blocking diode.  Just look up and list what specific parts you want to use and you'll likely get some input and suggestions if something isn't right.

Edit:  You may also have to protect the gate to avoid exceeding the maximum V_GS.

[BjornM]

I'm studying the datasheet in detail now, and with a large enough output capacitor I think I can accept the sawtooth ripple.

Thanks for the tip about the resistor, will do a calc and see how it looks.

[BjornM]

So in principle you want to design a high current LDO regulator.
You dont need more than a reference, an OPAMP and a couple of capable MOSFETS to dissipate 60W max.
The regulation loop can be kept simple as both your supply and load are well-tempered.

Yes I found some schematics for that, but then learned that it is easy to get into oscillation problems with the OPAMP and the large MOSFET. Goes above my head, so I hesitate to try.

[Terry Bites]

There are buck boost converters of course. They will give you a constant voltage for inputs above and below a set vout without the use of fire. eg LM25118 webench.ti.com/power-designer/switching-regulator.

[T3sl4co1l]

Bulky?  The bulky solution is the 60W heatsink for a linear dropper!  SMPS are more complicated, yes, but well worth it.

Tim

[fourfathom]

Many previous-generation (pre MPPT) solar-panel regulators used a relay or pass-transistor that would cycle on and off to limit the battery voltage.  There is virtually no power dissipation in the controller.  Some of these did a moderately high-speed duty-cycle control, and others would cycle at a many-second rate.   This works well with lead-acid chemistry batteries, but probably not appropriate for Lithium. 

[boB]

To save money, space and point of failure.

But if I can't figure out a circuit, then that's what I might do.

The thing is, I can connect the panel directly to the battery without any apparent problem. But for a long term solution, I'd like to cut off the overvoltage.

It's going to be hard to get anything cheaper than something like this for $16 USD...

https://www.amazon.com/gp/product/B08DCH8K8M/

[strawberry]

10A charger SMPS like this
~20mm thickness

[inse]

It's going to be hard to get anything cheaper than something like this for $16 USD...

https://www.amazon.com/gp/product/B08DCH8K8M/

Questionable review...
And we don't know how it fits to his 18V battery.

I am not sure whether it is worth the effort to squeeze the last % of efficiency out of the cell with MPP tracking and buck/boost converting as there is no use for the energy once the battery is charged.
Maybe there is not even so much to cook off as the charge current drops once you reach terminal voltage and during charging, the cell's capability will limit the voltage.
Some measurements should prove the assumptions.
And reverse current protection should be considered.

[BjornM]

Some buck converters can cleanly pass through voltages equal to or less than their regulating set-point voltage with high efficiency and not requiring "switching".  Obviously the converter needs to have adequate power to run itself.

Thanks, this was very helpful, I didn't know that. Will have a look at this option.

[BjornM]

I am not sure whether it is worth the effort to squeeze the last % of efficiency out of the cell with MPP tracking and buck/boost converting as there is no use for the energy once the battery is charged.
Maybe there is not even so much to cook off as the charge current drops once you reach terminal voltage and during charging, the cell's capability will limit the voltage.

I agree on all points. MPPT is overkill, and current from solar panels is very low at full battery charge. But solar panels have a higher voltage at low temperature, so over voltage protection is absolutely needed.

[mariush]

2 15A+  bridge rectifiers and a mechanical relay or mosfet ?

bridge rectifiers are cheap, <1$ for 15A+ rectifiers, and they're easy to heatsink. You have 4 diodes but you only use 2 diodes out of the bridge rectifier, so each rectifier drops ~ 1.4v..2v

21v --->  [ --- diode 1 -- diode 2 --- ]  ----19.5v ----> [ --- diode 1 -- diode 2 --- ]  ---- 18v

If the input voltage is  > 19.5v  have both rectifiers in circuit.
If the input voltage is 19..20v, short out one rectifier using a relay or a mosfet, you'll drop 19..19.5v - 1.5v = ~ 18v
If the input voltage is <18v, short the second rectifier using a relay or a mosfet and you'll get input voltage..

Maybe use a microcontroller to add some histeresis so you don't turn on and off the relays every second... once it's engaged, it stays for at least 30s or something like that. With mosfets or solid state relays you could do it faster.

[Bravo]

Look on AliExpress for "10A 30A 6-60V Lead-acid Solar Storage Battery Charger Control Module Digital Overcharge Protection Control Switch LED indicator" ..............for less than $10--.